1. Field of the Invention
This invention relates to electrically controllable contractile/swelling synthetic muscles and robotic actuators and their method of operation. More particularly, this invention relates to encapsulated polyelectrolyte polymeric gels in electrolyic solutions capable of undergoing reversible expansion and contraction when appropriately stimulated, such as by an electric field in the range of a few volts per centimeter.
2. Description of the Prior Art
There are many computer-controlled actuators available for robotic and related applications. Examples of such actuators are PVDF actuators, piezoelectric actuators, such as PZT and Lithium-Niobate, electroheological (ER) fluid actuators such as glass sphere/silicone oil actuators. Other common actuators are linear motors, electro-magnetic actuators, hydraulic actuators, pneumatic actuators, and explosive-type actuators. These prior art actuators suffer from various disadvantages, such as size, complexity, high weight and weight/displacement, large power requirements, and high material costs.
It is known that certain co-polymers may be chemically contracted and expanded in electrolytic solutions by varying the degree of ionization of the solution, or the pH. As originally reported by W. Kuhn, B. Horgitay, A. Katchalsky and H. Eisenberg, "Reversible Dilation and Contration By Changing The State of Ionization of High-Polymer Acid Networks," Nature, Vol. 165, No. 4196, pp. 514-516, (1950) a three-dimensional network, consisting of polyacrylic acid can be obtained by heating a foil of polyacrylic acid containing a polyvalent alcohol such as glycerol or polyvinyl alcohol. The resulting three-dimensional networks are insoluble in water but swell enormously in water on addition of alkali, and contract enormously on addition of acids. Linear reversible dilations and contractions of the order of more than 400 percent have been been observed. Furthermore, the ultimate structural deformation (swelling or collapsing) of these gels is homogeneous in the sense that, for example, for a long cylindrical gel, the relative changes of the length and the diameter are the same. Similar properties are exhibited by polymethacrylic acid cross-linked cross-linked by divinyl benzene copolymerized in methanol.
The use of chemically stimulated pseudo-muscular actuation for chemomechanical engines and turbines was originally discussed by I. Z. Steinberg, A. Oplatka, and A. Katchalsky, "Mechano-Chemical Engines, Nature, Vol. 210, No. 5036, pp. 568-571, (1966). Applications of polyelectrolyte gels driven by solvent substitution is discussed by D. Caldwell and P. Taylor, "Chemically Stimulated Pseudo-Muscular Actuation," International Journal of Engineering Science, Vol. 28, No. 8, pp. 797-808, (1990).
The same effect can be obtained electrolytically. Application of a voltage across the polymer gel causes a pH gradient to evolve between the electrodes. For example, the polymeric fibers may be filled with platinum by alternatively treating them with solutions of platinic chloride and sodium borohydride. A reversible expansion and contraction of the fiber is obtained with the application of an electric field. Direct motion control of these polymeric muscles with position and velocity feedback is feasible. The behavior of polymeric gels in an electric field is discussed by T. Tanaka, I. Nishio, S. Sun and S. Ueno-Nishio, "Collapse of Gels in an Electric Field," Science, Vol. 218, pp 457-469, (1982). In principle, the devices of the prior art need have only one moving part, the actuating gel itself. There is not the attendant weight and complexity of electric motors or hydraulic pumps and actuators. All that is required is an electric field of the order of a few volts per centimeter. The major disadvantages of such devices are that, in general, the response times of these gels are much longer than conventional actuator components, and there is the inconvenience that the gel must be contained within a solvent bath.
The above-mentioned disadvantages are overcome in the present invention by containing the gel and its bath in a container. The novel devices of the present invention employ either rigid containers, or flexible and foldable membranes as required to confine the bath.